1. Field of the Invention
This invention relates to energetic thermoplastic elastomers which are useful as binders of high-energy compositions, such as propellants, especially rocket propellants and gun propellants, explosive munitions, gas generants of vehicle supplemental restraint systems, or the like, and to methods for synthesizing the same.
2. Description of the Related Art
Solid high-energy compositions, such as propellants, explosives, gasifiers, and the like comprise solid particulates, such as fuel particulates and/or oxidizer particulates, dispersed and immobilized throughout a polymeric binder matrix.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the xe2x80x9cpot life.xe2x80x9d Disposal of a cast, cross-linked propellant composition is difficult, and usually is accomplished by burning, which poses environmental problems. Furthermore, current state-of-the-art propellant compositions have serious problems that include their use of nonenergetic binders which have lower performance and high end-of-mix viscosities.
In view of the inherent disadvantages associated with the use of cross-linked elastomeric polymers as binder materials, there has been considerable interest in developing thermoplastic elastomers suitable as binders for solid, high energy compositions. However, many thermoplastic elastomers fail to meet important requirements expected of propellant formulations, particularly the requirement of being processable below about 120xc2x0 C., it being desirable that a thermoplastic elastomeric polymer for use as a binder in a high energy system have a melting temperature of between about 60xc2x0 C. and about 120xc2x0 C. The melting temperature is desirably at least about 60xc2x0 C. because these compositions may be subject to somewhat elevated temperatures during storage and transport, and significant softening of the compositions at such elevated temperatures is unwanted. The setting of the melting temperature at not more than about 120xc2x0 C. is determined by the instability, at elevated temperatures, of many components which ordinarily go into the compositions, particularly oxidizer particulates and energetic plasticizers. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing. Thermoplastic elastomers typically obtain their thermoplastic properties from segments that form glassy domains which may contribute to physical properties adverse to their use as binders. Cross-linkable thermoplastic elastomers are block copolymers with the property of forming physical cross-links at predetermined temperatures. One thermoplastic elastomer, e.g., Kraton, brand TPE, obtains this property by having the glass transition point of one component block above room temperature. At temperatures below 109xc2x0 C., the glassy blocks of Kraton form glassy domains and thus physically cross-link the amorphous segments. The strength of these elastomers depends upon the degree of phase separation. Thus, it remains desirable to have controlled, but significant, immiscibility between the two types of blocks, which is a function of their chemical structure and molecular weight. On the other hand, as the blocks become more immiscible, the melt viscosity increases, thus having a deleterious effect on the processability of the material.
Above-mentioned U.S. Pat. No. 4,361,526 proposes a thermoplastic elastomeric binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystal structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. The ""526 patent states that this polymer is processed with a volatile organic solvent. Solvent processing is undesirable inasmuch as the dissolved composition cannot be cast in a conventional manner, e.g., into a rocket motor casing. Furthermore, solvent-based processing presents problems with respect to removal and recovery of solvent.
The preparation of energetic thermoplastic elastomers prepared from polyoxetane block copolymers has been proposed in U.S. Pat. No. 4,483,978 to Manser and U.S. Pat. No. 4,806,613 to Wardle (xe2x80x9cthe ""613 patentxe2x80x9d), the complete disclosures of which are incorporated herein by reference to the extent that these disclosures are compatible with this invention. According to the latter, these materials overcome the disadvantages associated with conventional cross-linked elastomers such as limited pot-life, high end-of-mix viscosity, and scrap disposal problems.
The thermoplastic materials proposed by the ""613 patent involve elastomers having both (A) and (B) blocks, each derived from cyclic ethers, such as oxetane and oxetane derivatives and tetrahydrofuran (THF) and tetrahydrofuran derivatives. The monomer or combination of monomers of the (A) blocks are selected for providing a crystalline structure at usual ambient temperatures, such as below about 60xc2x0 C., whereas the monomer or combination of monomers of the (B) blocks are selected to ensure an amorphous structure at usual ambient temperatures, such as above about xe2x88x9220xc2x0 C. Typical of these materials is the random block copolymer (poly(3-azidomethyl-3-methyloxetane)-poly(3,3-bis(azidomethyl)oxetane), also known as poly(AMMO/BAMO). These block copolymers have good energetic and mechanical properties. Additionally, the block copolymers can be processed without solvents to serve as binders in high performance, reduced vulnerability explosive, propellant, and gas generant formulations. Advantageously, the block copolymers exhibit good compatibility with most materials used in such energetic formulations.
However, the block copolymers known in the art suffer from disadvantages that are a consequence of the short linking groups connecting the blocks. More specifically, the short linking groups attribute relatively low softening temperatures to the copolymers. In tactical and other environments in which the binder is exposed to extreme environmental conditions, the binder should be capable of maintaining their structure integrities without creeping or slumping, and be characterized by a reasonable modulus at about 60xc2x0 C. or above. While the energetic binders disclosed in the ""613 patent generally satisfy the processing requirements, they tend to soften unacceptably at elevated temperatures that sometime are encountered in tactical and similar uses.
One proposed solution to addressing this problem and imparting desired high temperature attributes to the energetic binder is to select hard blocks, i.e., A blocks, having melting temperatures well above 60xc2x0 C. However, the higher softening temperatures of such A blocks deleteriously affects the processability of the binder by requiring higher and sometimes dangerous processing temperatures. Although solvents may be used to improve processability, the introduction of solvents limits the size of the articles that can be made and increases the complexity and costs of the process.
Another desired attribute of energetic binders is that the binders maintain strength, toughness, and strain capability at extremely low temperatures, preferably below about xe2x88x9240xc2x0 C. The polyethers used as the soft blocks, i.e., B blocks, in energetic thermoplastic elastomer binders tend to possess glass transition temperatures Tg in the range of xe2x88x9215xc2x0 C. to xe2x88x9230xc2x0 C. Below these temperatures, the thermoplastic elastomer binders become brittle and lack sufficient toughness and strain capability. While plasticization of the soft B block potentially could be a solution to lowering Tg of the thermoplastic elastomer, all attempts at plasticizing the B block have been found to require unacceptable plasticizer-to-polymer ratios, making the binder unusable at ambient and higher temperatures.
It would, therefore, be a significant advancement in the art to provide energetic thermoplastic elastomer binders that are solid at room temperature and exhibit the excellent mechanical properties and processability of the materials disclosed in the ""613 patent, while both possessing sufficiently high elevated temperature modulus and resistance to slump and creep while at the same time having a lower glass transition temperature or ability to be plasticized without adversely affecting high temperature properties.
The present invention relates to a thermoplastic elastomer that addresses the aforementioned problems associated with the related art and realizes the advancement expressed above.
The present invention includes the provision of an energetic thermoplastic elastomer binder that is in a solid state at room temperature and has A blocks and B blocks connected via linking groups derived from a difunctional urethane oligomer diol. The A blocks are crystalline at temperatures below about 60xc2x0 C. and may be one or more polyether(s) derived from one or more monomers of oxetane derivatives and/or tetrahydrofuran derivatives. The B blocks are amorphous at temperatures above about xe2x88x9220xc2x0 C. and may be include one or more polyether(s) derived from one or more monomers of oxetane and its derivatives, tetrahydrofuran and its derivatives, and/or oxirane and its derivatives. To effect linking, the A blocks and B blocks are capped with isocyanate moieties of diisocyanates. The isocyanate moieties of adjacent blocks are linked with an oligomer having two functional moieties that are reactive with an isocyanate moiety of the diisocyanate, hereinafter xe2x80x9ca difunctional oligomer.xe2x80x9d The structure and length of the difunctional oligomer may be varied to tailor the properties of the resulting thermoplastic elastomer.
The present invention also relates to a method for the preparation of the above-described energetic thermoplastic binder of this invention. In accordance with the principles of this invention, hydroxyl-terminated A blocks, which are crystalline at temperatures below about 60xc2x0 C., and hydroxyl-terminated B blocks, which are amorphous at temperatures above about xe2x88x9220xc2x0 C., are end-capped with a diisocyanate. The diisocyanate preferably has one isocyanate moiety which is more reactive, preferably at least about five times as reactive, with the terminal hydroxyl group of each of the blocks than the other isocyanate moiety, whereby the more reactive isocyanate moiety tends to react with the terminal-hydroxyl groups of the blocks, leaving the less reactive isocyanate moiety free and unreactive. The end-capped A blocks and the end-capped B blocks are mixed together at approximately the stoichiometric ratios that the blocks are intended to be present in the energetic thermoplastic elastomer. The mixture is reacted with a chain extender, i.e., an oligomeric linking compound having two functional groups which are sufficiently unhindered to react with the free and unreacted isocyanate groups of the end-capped blocks. In this manner, the end-capped blocks are linked, but not cross-linked, to form a thermoplastic polymer.
Propellants, especially rocket propellants and gun propellants, explosives, gas generants, or the like, containing the above-discussed energetic thermoplastic elastomer binder or made by procedures including the above-discussed method are also provided.